1. Field of the Invention
This invention relates to a method and apparatus for measuring deviation from straightness, or wall curvature or axial curvature, of an elongated member, such as, for example, a tube, using ultrasonic pulse-echo techniques.
2. Description of the Prior Art
A wide variety of ultrasonic testing and measuring systems exist which are capable of either inspecting materials or workpieces for hidden flaws, or gauging the thickness or diameter of a workpiece. Typically, in ultrasonic pulse-echo apparatus, an electronic pulse generator provides pulses to an ultrasonic transducer acoustically coupled to the surface of the material or workpiece. In response to the electrical signal from the pulse generator, the transducer transmits a search signal which propagates through the coupling medium and into the workpiece. When an acoustic discontinuity, as for example, a flaw, lamination, or wall surface, is encountered in the workpiece by the search signal, a reflection or echo signal is produced which is detected by the transducer and converted into an electrical echo signal. The electrical search and echo signals from the transducer are generally used either to respectively trigger and stop a clock circuit or to provide a coordinate display on a cathode ray tube. From the time elapsed between the transmission of a search signal and the receipt of an echo signal by the transducer, it is possible to determine the distance between the face of the transducer and the acoustic discontinuity represented by the flaw, lamination or wall surface of the workpiece. A typical prior art system for measuring the thickness and location of a flaw in a workpiece, as described above, is found in U.S. Pat. No. 3,570,279, issued to D. H. Davies on Mar. 16, 1971.
U.S. Pat. No. 3,780,442, issued to W. M. Gresho on Dec. 25, 1973 discloses measuring the axial curvature of an elongated member, such as, for example, a tube. A plurality of wall curvature gauges are both mounted independent of each other on a rigid carrier and arrayed equiangularly about the axis of the carrier to form a compound gauge. Each curvature gauge comprises a subcarrier having a radially movable probe mounted on the subcarrier between two radially extending, spaced apart, fixed feet, and a linear variable differential transformer (LVDT) coupled to the probe. When the gauge is positioned in the tube, the tips of the fixed feet contact the referenced surface of the tube to form a longitudinal reference line therebetween. Any deviation from the reference line by the tip of the probe, which is in contact with the referenced surface of the tube, will cause the associated LVDT to generate a corresponding electrical signal thereby to indicate wall curvature along a particular wall area. Each of the LVDT generated electrical signals are proportionately weighted and concurrently processed to provide a single continuous output voltage signal indicating the axial curvature of the tube. The Gresho patent, however, does not disclose nor teach non-contact method of apparatus, or the use of ultrasonic pulse-echo techniques for effecting an axial curvature measurement of an elongated member.
The problem, therefore, is to provide method and apparatus which will measure the axial curvature of an elongated member while avoiding contact of the measuring apparatus with a surface of the member.